1. Field of Invention
This invention relates to capacitance type, non-contact displacement and/or vibration measuring instruments.
More particularly, the invention relates to a novel sensing head assembly for such instruments and to a new and improved method of maintaining calibration of such instruments employing the novel sensing head assembly.
2. PRIOR ART PROBLEM
A known, prior art capacitance type, non-contact displacement and vibration measuring instrument is described in U.S. Pat. No. 3,048,775 issued Aug. 7, 1962 to R. Calvert and entitled "Apparatus for Measuring Small Distances". The capacitance type, non-contact displacement measuring instrument described in U.S. Pat. No. 3,048.775, as well as in other known instruments of the same type, require either a much larger sensor surface area relative to the desired measurement range, or the use of an electronic linearizer in the instrument processing circuitry in order that the instrument provide a linear output over a comparatively large physical displacement range. For example, one such known instrument requires that the probe sensing electrode area be in the order of 1.5 square inches in area for each inch of displacement sensing range. The relatively large probe area/displacement range size requirement often leads to serious restrictions or limitations in the use of such instruments in experimental and commercial applications. Another known device requires the use of an electronic linearizer in the processing circuitry used in the instrument in order that a more favorable relationship between sensor size and measurement range can be obtained. However, the use of the linearizer in the processing circuitry introduces calibration problems, operational difficulties, circuitry complexity and increased cost.
With either of the above briefly discussed known capacitance type, non-contact displacement and vibration measuring instruments, it is customary to employ a "driven guard" sensing probe assembly such as that shown in FIG. 1 of the drawings in order to prevent errors that otherwise would be introduced due to such variables as connector cable capacitance and stray circuit capacitance. Electrical design considerations dictate that in order to achieve the desired result of cancellation of these error producing factors, the "driven guard" ring must be operated at a one to one voltage and phase relationship relative to the excitation signal applied to the central sensor probe electrode. Thus, it is customary to apply the same excitation signal to both the central sensor electrode probe member and the "driven guard ring" that surrounds the central sensor electrode probe member. This known practice tends to minimize the classical fringe effect present in a parallel plate capacitor system and results in a substantially homogeneous electrostatic field across the central sensing electrode probe member. The difficulty with the system arises in measurement applications involving rather large physical displacements in excess of 100 milli-inches (0.1 inches) due to the fact that when operating such known systems at large ranges in excess of the 100 milli-inches, divergence of the electrostatic field at the driven guard ring which surrounds the central sensing electrode probe, introduces non-linearities which are not overcome short of introducing additional circuit complexity in the processing circuitry of the instrument. To overcome this problem and at the same time extend the range of physical displacement which can be measured accurately with the instrument without undue additional complexity and additional cost, the present invention was devised.